Estimation of NO x and SO 2 Emissions from Sarnia, Ontario using Mobile-MAX-DOAS and a NO x -Analyzer

Abstract

Abstract. Sarnia, ON experiences pollutant emissions disproportionate to its relatively small size. The small size of the city limits traditional top-down emission estimate techniques (e.g., satellite) but a low-cost solution for emission monitoring is Mobile-MAX-DOAS. Measurements were made using this technique from 21/03/2017 to 23/03/2017 along various driving routes to retrieve vertical column densities (VCDs) of NO2 and SO2 and to estimate emissions of NOx and SO2 from the Sarnia region. A novel aspect of the current study was the installation of a NOx analyzer in the vehicle to allow real time measurement and characterization of near-surface NOx/NO2 ratios across the urban plumes, allowing improved accuracy of NOx emission estimates. Confidence in the use of near-surface measured NOx/NO2 ratios for estimation of NOx emissions was increased by relatively well-mixed boundary layer conditions. These conditions were indicated by similar temporal trends in NO2 VCDs and mixing ratios when measurements were sufficiently distant from the sources. Leighton ratios within transported plumes indicated peroxy radicals were likely disturbing the NO-NO2-O3 photostationary state through VOC oxidation. The average lower limit emission estimate of NOx from Sarnia was 1.60\u2009±\u20090.34 tonnes\u2009hr−1 using local 10\u2009m elevation wind-speed measurements. Our estimates were larger than the downscaled annual 2017 NPRI reported industrial emissions of 0.9 tonnes NOx\u2009hr−1. Our lower limit estimate of SO2 emissions from Sarnia was 1.81\u2009±\u20090.83 tonnes SO2\u2009hr−1, equal within uncertainty to the 2017 NPRI downscaled value of 1.85 tonnes SO2\u2009hr−1. Satellite-derived NO2 VCDs over Sarnia from the Ozone Monitoring Instrument (OMI) were lower than Mobile-MAX-DOAS VCDs, likely due to the large pixel size relative to the city’s size. The results of this study support the utility of the Mobile-MAX-DOAS method for estimating NOx and SO2 emissions in relatively small, highly industrialized regions especially when supplemented with mobile NOx measurements.